Fine edged blade



Oct. 20, 1931. T. H. FROST 1,827,871

FINE EDGED BLADE Filed Nov. l0, 1950 //V VE/V 717R Patented Oct. 20, 1931 PATENT OFFICE THOMAS H. FROST, OF CAMBRIDGE, MASSACHUSETTS FINE EDGED BLADE Application filed November 10, 1930. Serial No. 494,591.

This invention relates to fine edged blades of ferrous alloys which may 'be hardened by the process of nitriding. In one aspect it comprises a novel blade of such alloy characterized by an elongated narrow nitrided area in which the cutting edge is included. In another aspect it comprises an improved method of producing such blades.

An important field of use of my invention is the product-ion of safety razor blades of the thin flexible type which are adapted for use in holders wherein they are clamped for support. A nitrided blade of this character 1s disclosed in my copending application Serial No. 455,366 filed May 24, 1930, and in still another aspect the present invention constitutes a development or carrying forward of the invention therein disclose The material I prefer to employ in the construction of the novel blade of my invention is an alloy of steel containing a substantial amount of aluminum, usually combined with chromium or molybdenum or tungsten and this I will refer to under the name Nitral- 10y by which it is commercially designated. When such metal is exposed to ammonia fumes under conditions of high temperature it becomes nitrided upon its exposed surfaces and to a slight distance below its surface and is thereby rendered extremely hard and brittle. Attempts to use this material in the manufacture of fine edged blades have been unsuccessful prior to my earlier invention, partly on account of the fact that an expansion of the material takes place in the nitriding operation which warps or distorts the blank to an objectionable extent. Moreover,

blades nitrided throughout their area have been too brittle for practical purposes. This has been particularly true of suchan article as a flexible safety razor. blade which is com-' monly maintained in a position of pronounced curvature "when in use and must therefore be capable ofwithstanding apprecompletely nitrided is likely to shatter when bent between the fingers and become a men? body of the blade offsets the fragility of its 'cutting edge.

ciable bending. A razor blade of nitralloy' little more than the bevelled portion of the blank or the vertex of the cutting edge, then the blade as a .whole, on account of its softer body portion, is rendered durable and capable of such flexing as is normally imposed upon safety razor blades. While the actual cutting edge is rendered extremely hard and somewhat brittle the more flexible tougher cutting edge.- I have further discovered that a blade of this characteristic may be produced by bevelling an edge of each blank and then, prior to the nitriding step, stacking the bevelled blanks so as to protect the bodies and expose the bevelled edges only thereof. By this procedure it is possible to produce blades in which the nitriding operation is confined to the bevelled portions which project from the sides of the stack in a slightly separated or spaced condition, whereas the engagement of contact between the fiat faces of the blanks tends to prevent the entrance of ammonia and consequent nitriding of any substantial area within the While I believe it is possible by insuring suitable conditions of pressure and flatness to stack the blanks for nitriding in the manner outlined, I prefer to employ spacing shims between adjacent blades and by employing shims of a non-nitriding material such, for example, as nitralloy shims already nitrided or shims of a non-ferrous metal, entrance of ammonia gas between the blanks is more effectively. prevented.

The method of my invention, if preferred, may be characterized by the employment of shims plated with certain non-ferrous metals, for example, tin. When the stacked blades are separated by tin-coated shims the ammonia gas used in the nitriding operation is apparently prevented from reaching the adjacent faces of the stacked blanks by the action of the vaporized tin and a particu-: larly clean cut zone of nitriding may be thus secured. I

' These and other features of myinvention willbe best understood and appreciated from the. following description of its application to the manufacture of safety razor blades as 1 illustrated in the accompanying drawings, in which:

Fig. 1 is a view in perspective of a blade bl nk,

Fig. 2 is a similar view of the blank with roughly bevelled edges.

Fig. 3 is a View in perspective of a shim. Fig. 4 is a View in perspective of a stack of blanks ready for the nitriding step.

Fig. 5 is a sectional view on the line 55 of Fig. 4, and

Fig. 6 is a View in perspective of a finished blade,

It will be understood that my, invention is of general application to fine edge blades for all uses, although a most important field of use is in the manufacture of safety razor blades and. its application to this field is herein illustrated.

In carrying out my invention a flat blank of the required shape is first produced. Such a blank 10 is shown in Fig. 1, having been punched or died out of thin sheet nitralloy and coinciding in its dimensions to that desired in the finished blade. The blank is internally apertured, as by a slot 12, to receive the blade. positioning elements of the holder and is provided with notches 14 at each corner to avoid corner cap pressure.

The material employed for the production of the blank may advantageously be of either of the followlng analyses:

- Element Analysis A Analysis B Carbon 36 23 Mm 51 51 Silicon 27 20 Aluminum i 1. 23 1. 24 Chromium l. 49 1. 58 Sulphur 010 011 Phosphorus. 013 .011 Molybdenum 18 20 .Theanalyses above set forth are typical of a satisfactory nitralloy material for the.

manufacture of fine edge blades but nitralloy of other similar analyses may be successfully used.

The next step in practicing the method of my invention consists in rough grinding the longitudinal edges of the blank to form a bevel 16. The formation of the bevels 16 is the first step in the production ofthe cutting w edge and they are subsequently smoothed and sharpened as will presently appear.

The roughbevelled blanks are next stackedflatwise and securedin such position for the nitriding operation. As herein shown, 1' prefer to interpose a shim between adjacent blanks project in'spaced relation from' the,

side surfaces of the stack, bein wholly exposed while the body of each lilank is enclosed and completely shielded by its engagement with the face of adjacent upper and lower shims. The shims 18 may be of copper, tin or other non-nitriding material or may be plated or otherwise coated with such material. As herein shown, I employ a steel shim having its opposite tinned faces.

The blanks with interleaved shims may be clamped in stack form in aily convenient manner, but as herein shown, the stack is 'enclosed betweena relatively thick bottom plate 20 and a relatively thick top plate 22 and the assembled pieces are clamped together by bolts 24 which pass through the plates, blanks and shims and are provided with, nuts for subjectingthe stack tothe desired pressure.

Having arranged the blanks in stackform,

these circumstances a hard nitrided case will.

be" formed upon the surface of the bevels 16 of the blank but nowhere else. This case may extend entirely through the material of the bevel or it may extend merely a few thousandths of an inch below the surface thereof but in any case the vertex of the blade is completely nitrided.

At the conclusion of the nitriding step the stack is removed from the furnace and allowed to cool. Thereupon the clamping bolts are released and the blades and shim's separated. It will be found that a case of suilicient depth is formed to enable the rough bevel 16 of the blank to be finish ground, honed and stropped to produce the smooth bevel 17 of the finished blade 11 shown in Fig. 6. As already intimated, the elongated narrow nitrided zone defined by the bevelv 17 in the finished blade is of extreme hardness and. although it is also brittle, it is-carried by the un-nitrided soft and flexible body of the blade so that as a complete article the blade isdurable and serviceable.

It will be noted that the rough grinding step produces a bevel 16 which approximates very closely the shape of the finished edgeand presents a relatively thin zone for the nitrid-' ing operation. The rough grinding step may be effected rapidly since the material to be \removedis relatively soft. The thin zone which is thereby produced is particularly well adapted to be efficiently nitrided since the depth of penetration required to form .a completely nitrided cutting edge is reduced almost to a minimum. At the conclusion of the nitriding operation the rough beveled portion of the edge is brought to a fine, keen cutting edge by the finish grinding or otherwise smoothing. ,Here again an advantage of my improved process is apparent, for while there is a tendency to feather or form a wire edge in grinding the softer unnitrided material, the hard, brittle, nitrided material can be ground accurately or smoothed with no tendency to form a wire edge. In Fig. 6 the finish ground sharp cutting edge is indicated by reference character 17.

While I have herein referred to nitralloy as being a suitable and preferred material for the construction of blades in accordance with the novel method of my invention, the use of other ferrous alloys capable of being hardened by the process of nitriding is contemplated and would be within the scope of my invention. 1

Having thus described my invention, what I claim as new and wish to secure by Letters Patent of United States is: y

1. A fine edged blade of a nitridable ferrous alloy bevelled to form a cutting edge, and

nitrided only in the. narrow area defined by the bevel of the blade. v

2. A fine edged blade of a nitridable ferrous alloy having a cutting edge, and nitrided only in an elongated narrow area which includes said cutting edge.

3. A safety razor blade of a nitridable ferrous alloy, internally apertured to receive positioning means, having an exterior cutting edge and being nitrided only in a narrow area remote from its apertures and including the cutting edge.

4. A safety razor blade of a nitridable ferrous alloy, having a bevelled cutting edge and being provided with a nitrided case which includes the vertex of the bevel and a narrow 6. The method of making fine edged blades,

which consists in producing flat blanks of a nitridable ferrous alloy. rough bevelling an.

edge of each blank, stacking the blanks under pressure, disposed face to face with their bevelled edges only exposed. nitriding the exposed edges, and sharpening the nitrided edges. I

7. The method of making fine edges blades, which consists in producing flat blanks of the required shape, from a nitridable ferrous alloy, bevelling an edge of each blank, stacking the blanks with interposed shims of such size as to maintain the bevelled edges in spaced projecting bevelled edges.

8. The method of making fine edged blades,

which consists in providing flat blanks from a nitridable ferrous alloy, bevellin-g anedge of each blank, interleaving the blanks with shims presenting non-ferrous contact faces and of such size as to maintain the bevelled edges in spaced relation, and then nitriding the bevelled edges of the blanks.

9. The method of making fine edged blades, which consists in stacking a series of nitridable bevel-edged blanks with interleaved shims of non-nitriding material, the shims terminating within the bevelled edges of the blanks thus exposing them in spaced relation, and then nitriding said exposed edges.

10. The method of making blades, which consists in stacking a series of nitridable bevel-edged blanks with interleaved shims having tin-coated surfaces, and exposing the stack to the action of ammonia gas under conditions of high temperature, thus nitriding the narrow bevelled areas only of the blades.

11. The method of making safety razor blades, which consists in stacking flat, nitrida-' ble, bevel-edged blade blanks in alternation with tin-coated shims of less width than the blanks, exposing the stack to the action of ammonia gas under conditions of temperature sufficiently high to vaporize a portion of the tin from the shims thus hardening the bevelled portions of the blanks, and finally finish grinding the bevelsto complete the blades.

12. A nitralloy safety razor blade having a relatively soft tough body of unnitrided nitrallov. and a narrow marginal zone which is nitrided and therefore hard and rust-re sistant and in which the cutting edge is formed.

13. The method of making fine edged blades from a nitridable ferrous alloy, which consists in rough grinding the edge of the blade blank to form a bevel of the approximate shape desired. then nitriding the blank in an area including said beveled portion, and finally finishing the nitrided beveled portion to form a keen cutting edge.

14. The method of making safety razor blades of a nitridable ferrous alloy, which ins consists in rough grinding the edge of the blade blank to form a bevelled portion of approximately the desired shape, nitriding the beveled portion while protecting the body of the blank against nitriding, and finally smoothing the nitrided bevelled portion by removing sulficient nitrided material to form a fine cutting edge..

Signed at Boston,

day of November, 1930.

- I THOMAS H. FROST.

. Massachusetts, this 8th 

